Wastewater containing organic substances are generally treated by a treatment system using organisms (bacteria). The treatment system using organisms produce a large quantity of excess sludge. In pursuit of realizing a recycle-based society, waste reduction and recycling has been promoted in Japan. The proportion of organic and inorganic sludge in the waste is high, and about 70% of the sludge is organic. Although recycling of organic sludge as, e.g., compost has come to be studied in public sludge treatment facilities or big business establishments, very little is recycled in small scale facilities. Since space is running out in waste landfills and finding a site for a new waste landfill is difficult, the cost of sludge treatment has been increasing year by year. It has therefore been demanded to develop a wastewater treatment system for reducing sludge production conveniently and economically.
In the light of the above circumstances, wastewater treatment facilities have proposed techniques for controlling or reducing sludge production. The proposals so far made typically include biological processes in which flocs of organisms are added to a bioreactor and processes in which part of produced sludge is solubilized by a physicochemical method, and the resulting solution is returned to a bioreactor to be treated biologically. The former processes have been empirically embodied making use of assimilation and secreted enzymes of Bacillus subtilis or photosynthetic bacteria. Recently proposed techniques belonging to the former processes include a sludge reduction technique using flocs of microorganisms obtained from piggery waste treatment as disclosed in patent document 1 (see below) and a sludge reduction system using thermophilic bacteria as disclosed in patent document 2 (see below). The methods for solubilizing the sludge used in the latter processes include those in which the cells of organisms in the sludge are physicochemically destroyed by milling (see patent documents 3 to 5 shown below), ozonization (see patent documents 6 to 9 shown below), ultrasonication (see patent documents 10 and 11 shown below), or water jetting (see patent documents 12 to 14 shown below).
Although these techniques are designed to achieve 10% to 20% reduction of sludge production compared with conventional technology, they have been put to little practical use for the following reasons. In the case of the biological processes, there is a problem that adding sludge-reducing bacteria into a bioreactor of a wastewater treatment plant brings about little change of the bacterial biota in the bioreactor or, even when the addition is continuous, the added bacteria do not predominate in the bacterial biota. Because of this problem, actual effects are uncertain. Moreover, reduction of solid matter typically including cellulose (e.g., paper) is difficult due to difficulty in decomposing in a bioreactor.
On the other hand, the techniques using a physicochemical process, such as milling, ozonization, or ultrasonication, involve an economical problem in practice because of general expensiveness of equipment. The milling method in which sludge is solubilized by the friction with beads, while providing high efficiency of finely comminuting the sludge in slurry, achieves only a low rate of solubilization and also has the problem that the scale of equipment is not easy to increase. The ozonization treatment, which belongs to accelerated oxidation processes, is high benefit not only because of ease and convenience of operation but also because of its ability to decompose persistent substances simultaneously. However, an ozone generator is expensive, and equipment for waste ozone disposal is separately required. The ultrasonication treatment attains a high rate of comminution and a high rate of solubilization but uses very expensive equipment and also needs a countermeasure against heat and noise generated by ultrasonication. In the water jet process, a sludge slurry is pressurized and jetted through nozzles into water to destroy the sludge by the cavitation caused by the difference in pressure. The water jet process shows high ability to finely comminuting a sludge slurry but requires a high power pump to comminute sludge effectively because the rate of sludge solubilization depends on the amount of cavitation generated.
Hence, the conventional techniques for controlling or reducing sludge production has their respective merits and demerits. A breakthrough in technology has thus been awaited.
In the light of the these circumstances, Applicant of the present invention previously proposed a process for reducing sludge production having high suppressive effect on sludge production and introducible into an existing wastewater treatment plant at low cost. The process is characterized by adding, to a system containing first activated sludge producing excess sludge, second activated sludge having a higher rate of autolysis than the first activated sludge (see patent document 15 shown below). This sludge reduction process was unsuitable for efficiently controlling a bioreactor of a wastewater treatment plant.    Patent document 1: JP 9-245A    Patent document 2: JP 9-234060A    Patent document 3: JP 11-300393A    Patent document 4: JP 2000-167597A    Patent document 5: JP 2000-325983A    Patent document 6: JP 9-234497A    Patent document 7: JP 11-90496A    Patent document 8: JP 2001-259678A    Patent document 9: JP 2001-327998A    Patent document 10: JP 2002-361281A    Patent document 11: JP 2003-200198A    Patent document 12: JP 2001-212599A    Patent document 13: JP 2001-314887A    Patent document 14: JP 2003-10890A    Patent document 15: Japanese Patent 3844771